1. Field of the Invention
This invention relates in general to three phase, bridge rectifiers used in dc drives, exciters and HVDC systems and more particularly to microprocessor based methods for controlling the firing angles for these rectifiers.
2. Description of the Prior Art
The six pulse bridge phase controlled rectifier is a widely used type of solid state power converter which is used in industry for converting a three phase ac input voltage to a variable dc voltage. The six pulse bridge phase controlled rectifier uses six thyristors as controllable power devices.
These thyristors, acting as power switches are turned on sequentially by the control circuit during each cycle of line voltage. The appropriate turn-on point (firing angle .alpha.) of the thyristors is determined by the amount of required power flow to the output of the rectifier and is adjusted by the control circuit during each cycle of line voltage.
Prior art control circuits for these converters have taken a number of variations. Most frequently the simplicity of the control circuit for a particular application has been the main consideration. However, the power circuit itself represents the major constituent of total system cost for HVDC transmission and variable voltage fed inverters. There is a need in the art for an advanced firing control scheme capable of meeting higher standard performances which can be used in a variety of applications.
Existing control circuits for the control of thyristors can be catagorized into two groups. The first is a control circuit which uses a conventional voltage controlled oscillator (VCO), a phase comparator and a ring counter to generate gating signals for thyristors. The second group uses a microprocessor to generate the gating signals for the control of the thyristors.
The first group will now be explained in more detail. The input control voltage is generally added to the phase comparator output voltage and applied to the input of the VCO. Thus, a variation of the delay angle is obtained by a temporary change in the VCO frequency. The ultimate change in the delay angle is proportional to the control signal. Control circuits of this design have an integral characteristic being that they can only operate in a closed loop system and the rate of change of firing angle is controlled rather than the firing angle itself. Consequently, in a closed loop system as shown by Sucena-Paiva and Freris, "Stabiity of Rectifiers with Voltage-Controlled Oscillator Firing Systems", Proc. IEE, Vol. 120, No. 6, pp. 667-674, June 1973 and Rumpf and Ranade, "Comparison of Suitable Control Systems for HVDC Stations Connected to Weak ac Systems. Pt. 1-New Control Systems", Vol. PAS - 91, pp. 549-555, 1972. There is a narrow stable operating region, as compared to a converter system with proportional control. Also, the response of such a system is slow, especially if operating in the inverting mode with a large delay angle. (Gupta, Venkatesan and Eapen, "A Generalized Firing Angle Controller Using Phase-Locked Loop for Thyristor Control," IEEE Trans. Ind. Electron. Cont. Inst., Vol. IECI-28 No. 1, pp. 46-49, Feb., 1981). Additionally, the offset and drift in the comparator amplifier changes the firing angle characteristics and the open loop gain of the converter is not constant, especially where the source impedance is considerable.
The second group of control circuits will now be analyzed. In this group the gating signals for the control of thyristors are generated by a microprocessor. For these controllers the converter is assumed to be fed by a strong source, i.e., without any source impedance and with a constant source frequency. The use of these converters with a weak ac system (with variable source frequency and considerable source impedance) may cause short circuits in the ac system and may also cause low frequency oscillations. Furthermore, the control system has a processing delay ranging from 15.degree. to 360.degree.. A disscussion for the 15.degree. end of the range may be found within an article by Olivier, Stefanovic and April, "Microprocessor Controller for a Thyristor Converter with Improved Power Factor," IEEE Tran. Ind. Electron and Cont. Inst., Vol. IECI-28, No. 3, pp. 188-194, Aug. 1981, where the processing delay is found to reflect the time required to perform the firing angle calculation. At the other end of the range around 360.degree., the processing delay is caused by the nature of the algorithm (Simard and Rajagopalan, "Economical Equidistant Pulse Firing Scheme for Thyristorized dc Drives," IEEE Trans. Ind. Electron. and Cont. Inst., Vol. IECI-22, No. 3, pp. 425-429, Aug. 1975.
Desired characteristics are lacking in both groups of control circuits. Specifically, the prior art is lacking in a fast response control circuit which can operate in an open loop as well as in a closed loop system without introduction of time lag. Please recall the first group presented only worked in a closed loop system. It is also desirable to have a constant open loop voltage gain, i.e., a linear relationship between the output voltage of the converter and the control input voltage. Moreover, it is preferable to extend the operation of the control circuit applications where a weak ac system, such as a small alternator, is feeding the converter. This necessitates the use of a firing angle control circuit that can operate properly over the unregulated frequency range and can compensate for source impedance, both in the synchronization and in the output voltge control circuits. Furthermore, to avoid the generation of undesired noncharacteristic harmonics, jitter free equidistant gating signals are conventionally preferred (Kimbark, "Direct Current Transmission," Wiley, 1971.)
According to Oliver, Stefanovic and April, "Microprocessor Controller for a Thyristor Converter with an Improved Power Factor," IEEE Tran. Ind. Electron and Cont. Inst., Vol. IECI-28, No. 3, pp. 188-194, Aug. 1981. There are several techniques for controlling a converter firing angle, such as: analog comparators, digital counters, phase-locked loops, etc. Pelly, "Thyristors Phase Controlled Converters and Converters, New York: Wiley, 1971; Oliver, Stefanovic and Jamil, "Digital Controlled Thyristor Current Source," IEEE Trans. Ind. Electron and Cont. Inst., Vol. IECI26, No. 3, pp. 185-191, Aug. 1979; Sen, MacDonald and Clarke, "A Novel Equidistant Pulse Control Scheme for Thyristor Converters, " Com. Elec. Eng. J., Vol. 3, No. 3, pp. 10-14, 1978. In almost all these schemes, a firing signal is generated when a time-varying signal becomes equal to a reference signal. The implementation can be analog as with a bias cosine method, Pelly, "Thyristors Phase Controlled Converters and Converters, New York: Wiley, 1971, or digital, as with a ROM look-up table and counters, Oliver, Stefanovic and April, "Microprocessor Controller for a Thyristor Converter with an Improved Power Factor," IEEE Trans. Ind. Electron and Cont. Ins., Vol IECI-26, No. 3, pp. 188-194, Aug. 1981, but the firing angle is always calculated with respect to a zero crossing point of the input-voltage waveform. Consequently, all these schemes may be grouped under the generic term of "absolute firing angle methods." However, since all these methods are not easily emulated by a microprocessor, a different approach more adaptable to software implementation, has been taken and a relative firing method has been developed.
The invention described herein overcomes the difficulties in emulating the analog absolute firing angle methods. Moreover, it provides capabilities that have not been shown to be possible when combined in any analog technique. These capabilities include 1/2.degree. response, operation with a weak ac system, and source impedance compensation.
To put things in perspective, we summarize the main features of the relativie delay angle approach described by Oliver, et al.
With the "relative firing angle method," the firing angle is controlled by lengthening or shortening the interval between two successive thyristor triggerings. Indeed, in the steady state, this interval, denoted by D, is equal to 60.degree.. A momentary decrease or increase of this interval, respectively, reduces or augments the firing angle .alpha., Oliver, Stefanovic and Jamil, et vir.
Also .alpha. is corrected by an amount .epsilon. every 360.degree. cycle to account for timing imperfections. This is done by starting a counter when the zero crossing signal arrives (the counter counts 1 MHZ clock pulses). At the time Q.sub.1 and Q.sub.6 are to be fired, the counter should contain what corresponds to .alpha.. The difference between the counter value and .alpha. is .epsilon.. Thus, by correcting .alpha. by .epsilon., timing imperfections are accounted for.
During transients, the interval .DELTA. (represented by the angle between two consecutive firing angles) cannot be negative and is limited to a minimum of 15.degree. by the execution time of the triggering routine. Maximum .DELTA. is 127.degree. to limit the size of the look-up table.
The software implementation of the relative firing angle approach can be summarized as follows: at the main program (loop), the reference voltage is read and the corresponding value is read from the arccosine table. Following this, the difference between this .alpha. and the value of .alpha. computed at the end of the last triggering interval is found. This difference is used to lengthen or shorten the next triggering interval.
The problems with the relative firing angle approach can be summarized as:
(1) Changes in the reference input voltage after correction are ignored until the end of the new gating interval which could be as long as 127.degree..
(2) If the change referred to above (in (1)) calls for a reduction of .DELTA., it will have to wait for possibly as long as 127.degree.. This degrades the performance.
(3) Since the .DELTA. counter is driven by a constant frequency source, operation with a weak ac system is impossible. Thus, this approach rules out operation with an unregulated source frequency.
(4) System response is very slow which causes performance degradation.